With a known monitoring device of such a type, in each circuit of two lamps to be switched-on in pairs there is arranged an excitation coil. Both excitation coils are disposed in a magnetic circuit, in the influence range of the latter there being arranged a magnetically actuated switch, namely a reed contact. The reed contact is operatively connected in a signal circuit, in which an indicator lamp serves as a signal transmitter. If both lamps which are to be monitored are simultaneously turned-on and are intact, the magnetic fluxes which are created by the currents in the excitation coils are neutralized or cancelled, and the magnetically influenceable reed contact remains open, so that the indicator lamp does not light up. To the contrary, if one of the two lamps are defective, then the magnetic flux which is produced by the excitation coil which is coordinated thereto is not compensated, and the magnetically influenceable reed contact is closed. In this case the indicator lamp is illuminated.
With this known monitoring device it is disadvantageous that the magnetically influenceable contact is comparatively expensive when it should be suited for a reliable operation over a long period of time. The assembling of this contact in the magnetic circuit with the excitation coils moreover has technical manufacturing disadvantages. The excitation coils must produce a magnetic induction that is sufficient for actuation of the contact; consequently they must have a sufficient winding number or number of turns. Thereby voltage drops can occur which are disturbing in the circuit of the lamps if the cross-section of the wire of the excitation coils is not selected sufficiently large, which can yet again increase the manufacturing costs. With the arrangement of the magnetically actuatable contact in the magnetic circuit, attention is carefully to be paid that the magnetic fluxes cancel and compensate, respectively, with intact circuits of the lamps. This also assumes lamp circuits which are to be switched-on in pairs with the same electrical values. In this manner the use of the known monitoring device can be limited.
The present invention thus is based on the task to provide a monitoring device avoiding the previously-mentioned disadvantages, which is suited for the monitoring of any arbitrary number of lamps to be simultaneously turned or switched-on, thus also being suited merely to one single lamp, which requires a comparatively small manufacturing expense with a reliable manner of operation, and particularly does not require an electromechanical actuatable contact as a circuit control element. Also the voltage drops caused by the monitoring device in the circuits of the lamps should be as low as possible.